Graphics Cards

Hi All, I know this is a Mac forum but over the years you have all proved very knowledgeable so hopefully you can help me answer a windows-based question!

I've been using a Mac for the past 3 years, having switched from a PC. It's now time to upgrade and, put simply, I cannot ignore the power of the Intel i7 920 (especially after some moderate over clocking) As much as I hate to admit it, the iMac simply can't compete with the computer I can build for £1000

Graphics for me are important, so I'm going to be getting the ATI HD 4870. My question is what is the difference between the Asus HD 4870 and Sapphire HD 4870? The Asus is £80 more expensive on eBuyer and I don't know why! They are both using the same chipset and they both have 1GB memory, so is one faster than the other?!

As ever, thanks for your help, and sorry if I have posted this in the wrong section.